Vertebrates detect many odorants with high sensitivity. An enormous amount has been learned about the chemical and electrical mechanisms underlying vertebrate olfactory transduction. A variety of GTP-binding proteins, enzymes, second messengers, and ligand-gated channels have been shown to function in transduction. There are 700 to 1000 genes that appear to code for odorant-receptor proteins. The remaining question is how this mass of transductory components is functionally organized across several million olfactory receptor neurons. The sheer number of components makes this question mathematically enormous and experimentally difficult. The nematode Caenorhabditis elegans presents an olfactory model that is tremendously less complex than the vertebrate system. Extensive evidence during the past five years has shown that the molecular components of olfaction in C. elegans closely parallel those of vertebrates. Such rapid progress has been enabled by the worm's small size and short generation time, and by the sequencing of most of its genome. However, in the absence of physiological studies, the exact functions of the molecular components of olfaction in C. elegans remain speculative. The proposed study will apply electrophysiological methods to the tiny identified chemosensory neurons of C. elegans. For the first time, the electrical basis of a sensory response in C. elegans will be demonstrated. The molecular functions implied by genetic and behavioral methods will be directly tested. The ability to record from these neurons will represent a significant enhancement of the existing methods and will broaden the usefulness of C. elegans as a model organism in neuroscience. C. elegans is a potential model for olfaction, chemotaxis, thermotaxis, mechanosensation, and responses to pheromones and light. The olfactory sense has a major role in regulating body hormonal state, emotional disposition, hunger, and social behavior. The work proposed here will advance understanding of the molecular and cellular processes which subserve sensory function.